The present invention relates to a pressurized fluid operated shifting mechanism for cooperation with a shift bar housing assembly for selectively shifting a change-gear mechanical transmission. More particularly, the present invention relates to such a fluid operated shifting mechanism of the "X--Y" type, and will be described in connection therewith.
Shift bar housing assemblies for change-gear mechanical transmissions typically comprise a plurality of generally parallel, independently axially movable shift bars or shift rails, each carrying a shift fork fixed thereto. Each shift bar or shift rail may be selected and moved axially to effect engagement/disengagement of a particular gear ratio. Such assemblies are well known in the prior art and may be better understood by reference to U.S. Pat. Nos. 4,455,883; 4,575,029; and 4,584,895, all of which are incorporated herein by reference.
Such shift bar housings may be manually controlled by the vehicle operator, by means of a shift lever. More recently, it is becoming increasingly popular to utilize automatically controlled pressurized fluid actuating shift actuators with such shift bar housings. In a pressurized fluid operated shift actuator, a shift finger is automatically moved in the X--X (rail selection) direction, in response to actuation of one fluid pressure device, then the shift finger is automatically moved in the Y--Y (gear engagement/disengagement) direction in response to actuation of another fluid operated device.
Shift bar housing assemblies utilizing pressure operated shift actuators to control each shift rail in an automatic or semi-automatic mechanical transmission are known in the prior art, as may be better understood by reference to U.S. Pat. Nos. 4,445,393 and 4,722,237, both of which are incorporated herein by reference.
The prior art fluid operated shift actuators for providing automatic and/or semiautomatic shifting of change-gear mechanical transmissions were not totally satisfactory. In such actuators, pressurized fluid was supplied at only a single fixed (regulated) fluid pressure (typically a relatively high pressure) which resulted in the same amount of force being applied to each shift rail, under all operating conditions. In other words, in the prior art actuators, there is a relatively high force applied to the shift rail to accomplish gear engagement (as is required), but the same relatively high force is also applied to the shift rail to accomplish gear disengagement. Such high force is not required for gear disengagement, and may be detrimental to the operation of the transmission. In many such transmissions, neutral (gear disengagement) is preselected, but the shift to neutral does not actually occur until the vehicle operator takes his foot off the accelerator, and the torque goes through zero, at which point the shift to neutral will occur. If the force being exerted to shift into neutral is too great, the shift into the gear disengagement position may occur before the torque passes through zero. In addition, there is the possibility of overshooting a selected position, as well as the possibility of undue wear to components such as shift forks and the like.
In response to the shortcomings of the prior art described above, there has been developed a dual pressure source type of fluid operated shift actuator, whereby a greater force is exerted on the shift rail moving into gear engagement and is exerted in moving to gear disengagement. See U.S. Pat. No. 4,928,544, assigned to the assignee of the present invention and incorporated herein by reference. Although the actuator assembly of the reference patent successfully overcomes the problems discussed above, the requirement for a dual pressure source and the additional solenoid valves required adds substantially to the overall cost and complexity of the shift actuator assembly.